Filament power supply circuit for tube testers



Jan. 18, 1966 a. A. TAYLOR, JR 3,230,417

FILAMENT POWER SUPPLY CIRCUIT FOR TUBE TESTERS Filed Jan. 16, 1961 OVERALL IMPEDANCE SERIES RESISTANCE fi HEATER RESISTANCE FIG. 2

HEATER RESISTANCE REACTANCE FIG. 4

INVENTOR E MER A. TAYLOR, Jr.

ATTORNEY United States Patent 3,230,417 FILAMENT POWER SUPPLY CIRCUIT FOR TUBE TESTERS Elmer A. Taylor, Jr., St. Joseph, Mich, assignor, by

mesne assignments, to Heath Company, St. Joseph,

Mich a corporation of Delaware Filed Ian. 16, 1961, Ser. No. 82,987 2 Claims. (Cl. 315106) This invention relates to a power supply and particularly to a power supply circuit for delivering a given value of current to a load having a variable impedance value such as, for example, the power supply circuit which provides the voltage and current requirements for the heaters of the various tubes to be tested on a tube tester.

In recent years tube manufacturers have developed a great many different types of tubes for use in series string applications, i.e., applications in which the tube heaters are connected in series with each other rather than in parallel. In this group of series string tubes, there are a rather large number of different heater voltages required. It is the usual practice in tube tester circuitry to provide a voltage tap on the power transformer for each value of heater voltage that is required. With the large number of different heater voltages required for series string tubes, the construction of the power transformer required becomes rather complex and costly by reason of the great number of voltage taps required. Additionally, new series string tube types are introduced from time to time which will undoubtedly have still different heater voltage ratings than those already in existence thus rendering the tube tester obsolete in a relatively short period of time.

The series string group of tubes includes the very well known 12.6 volt, 150 milliampere line of tubes; however, this line presents no problem since most of these can be tested on the single 12.6 volt heater switch position. The problem exists with regard to the large number of tubes requiring different heater voltages, nearly all of which I have observed, fall into one of three current requirement classificationsi.e., 300, 450 and 600 milliamperes. Since most fall into these few current categories and recognizing that new types to be introduced will likely also have one of these current ratings regardless of its voltage rating, it became clear that the provision of a constant current type power supply would be a far more satisfactory approach to the design of the heater power supply than by providing the necessary known voltages.

Accordingly, it is an object of this invention to provide a power supply capable of delivering a substantially constant current to a number of loads having different impedances which are connected one at a time to the power supply.

It is another object to provide a constant current power supply circuit for a tube tester which is capable of automatically delivering substantially the proper current to different tube heaters having a common current rating but different voltage ratings.

One of the features of the invention is that the use of the circuit described herein in a tube tester increases the up to date life of the tester so that the introduction of many new tube types does not render the tester obsolete in as short a time as with prior tube tester circuitry.

One of the advantages of the invention is the elimination of the necessity for the complex and costly provision of a large number of heater voltage taps on the heater supply transformer.

These and other objects, features and advantages will become apparent from a reading of the specification and claims taken in conjunction with the drawings, in which:

FIGURE 1 is a schematic wiring diagram of a circuit employing the principles of my invention,

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FIGURE 2 is a vector diagram illustrating the operation of the circuit of FIGURE 1,

FIGURE 3 is a schematic wiring diagram of a further circuit employing the principles of my invention, and

FIGURE 4 is a vector diagram illustrating the operation of the circuit of FIGURE 3.

Referring now to FIGURE 1, there is shown a portion of a tube tester power supply circuit having a source of alternating current 10 with a pair of terminals 10a and 1%. A load in the form of a heater element 11 of a vacuum tube 11a is connected to another pair of terminals 1201-1212. This heater then forms part of a series circuit comprising the heater 11, a resistance 13 and a switch 14 between the terminals 10a and 10b. The value of the resistance 13 is chosen to be very large compared to the value of the heater element 11 so that the current flowing through the element 11 is determined principally by the value of the resistance 13. In this manner, it will be possible to supply substantially the proper current value to the heater of any tube having a widely ditferent heater resistance from the tube 11a, which is connected to the terminals 12a-12b in place of the tube 11a, so long as these heater resistance values are very small compared to the resistance of the resistor 13. It will be appreciated that since substantially the proper value of current will pass through the heater that in accordance with Ohms Law, substantially the proper potential will automatically exist across the heater terminals.

Let us now consider a typical example where volts would be employed for the AC. source 10 of FIGURE 1 and it is desired to test a tube having a heater rating of 9.5 volts at 300 milliamperes such as the type known commercially as the 9CL8. In order to produce 300 milliamperes in the heater, the value of the resistor 13 and the heater 11 would have to be, according to Ohms Law The heater resistance when hot would be or 31.7 ohms. This value subtracted from the figure of 383 ohms would give us a value of 351.3 and let us therefore assume only for purposes of this example that the 9.5 volt rating is the median voltage for all known 300 milliampere series string tubes so that the 300 milliampere series resistor 13 will have the value of 351.3 ohms.

Now let us further assume that we desire to test another tube also having a 300 milliampere heater but with a heater voltage rating of 12.6 volts, such as the type 12SN7. The resistance of this heater when hot would be or 383 ohms.

or 42 ohms. This value added to the value of 351.3 ohms of the resistor 13 would result in a total resistance of 393.3 ohms across the AC. source terminals 10a-10b and therefore a current of or 292 milliamperes would flow through the heater of the 12SN7 tube. We thus see that the circuit of FIGURE 1 acts essentially as a constant current source since two tubes having considerably different resistance values, namely 31.7 and 42 ohms cause only a very small change of 2.66% in the flow of current when one is substituted for the other in the circuit.

Thus far we have been discussing only a heater current of 300 milliamperes. In order to test the 450 and 600 milliampere tubes, resistors 16 and 17 respectively are provided and are connected to appropriate taps on the switch 14 to be switched int-o the circuit in place of the resistor 13. The values of these resistors can be determined in accordance with the computations above for the resistor 13.

FIGURE 2 illustrates the vector diagram which shows the phase relationship between the resistance or impedance of the heater 11 and the series ballast resistor 13. It will be noted that these two Values are in phase since the currents through these two elements are also in phase. Although the resistance of the heater 11 is much smaller than that of the series resistor 13, the result of substituting a tube for the tube 11a which has a different heater resistance value from that of the tube 11a is to cause a direct relationship change in the total circuit impedance. The change in overall circuit impedance and therefore the change in current with a given change in heater resistance can be further minimized by employing a reactance device in place of the resistor 13 so that there is a phasedifference between the reactance and the heater resistance rather than an in-phase relationship as illustrated above. In this way, a greater range of heater impedances may be employed with a given value of series impedance r reactance with-out varying substantially from a desired predetermined current rating.

A circuit which accomplishes this is shown in FIGURE 3 in which like numerals designate like parts shown in FIGURE 1. In this circuit the capacitors 21, 22 and 23 replace the resistors 13, 16 and 17 respectively of FIG- URE 1.

FIGURE 4 illustrates the vector relationship of the series capacitive reactance or impedance and the heater resistance where it will be seen that the capacitive reactance leads the heater resistance by 90. This 90 phase relationship results in far less change in heater current for a given change in heater resistance than would be the case in the circuit in FIGURE 1 as will be evident from the following computations.

Considering again the heater rating of 9.5 volts of the 9CL8 tube as the design center from which to determine the reactance value of the capacitor 21, the total circuit impedance vector of FIGURE 4 would have to be a value of 383 ohms to allow 300 milliamperes to flow through the heater. As was determined above, the resistance of the 9CL8 heater when hot is 31.7 ohms.

Since Z =X, +R Where Z=t0tal circuit impedance X =capacitive reactance R=resistance and Z=383 ohms and R=31.7 ohms then X =382 ohms. Having the value of X the value of the capacitance can then be determined from the formula 1 C 2 rrfX where C: capacitance value f=frequency of the applied potential X =capacitive reactance.

For a frequency of 60 cycles and a capacitive reactance value of 382 ohms, the capacity value required would then be approximately 6.9 microfarads to produce 300 milliamperes through the 9CL8 heater.

The circuit of FIGURE 3 provides a very satisfactory arrangement for providing a given heater current to the heaters of various tubes having a common current rating but widely diiferent voltage ratings. This is made possible by the combination of the high reactance value of the capacitor relative to the heater resistance, coupled with Table I Percent Change from 300 Ratio of Voltage and Current 0 Htr. Res.

Rating The above computations and table apply for the 300 milliampere case. Similar computations can be made to determine the values of the capacitors 22 and 23 for the 450 and 600 milliampere cases respectively and to produce tables for these current values similar to the one above. 1

Since many changes could be made in the above circuit and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

I claim:

1. In tube tester apparatus for testing various types of vacuum tubes having heater elements of respective different current ratings, a circuit for supplying a substantially constant current to the heater element of a vacuum tube under test having a predetermined current rating comprising:

a source of alternating current;

a plurality of fixed reactance elements of different values, each having a single fixed reactance value at any given operating frequency;

switch means for selectively connecting the alternating current source and one of the reactance elements in series with the heater element of the vacuum tube being tested; and

the selected reactance element having a reactance value at the frequency of the alternating current which is much greater than the rated operating resistance of the heater element of the vacuum tube under test whereby the electrical current supplied to the heater element will be of a desired predetermined value which is substantially independent of the magnitude of the actual resistance of the heater element over a wide range of resistance values.

2. Tube tester apparatus for testing various 'types of vacuum tubes having heater elements of respective different current ratings whose resistances may vary over a wide range depending on the particular vacuum tube type under test, which particular vacuum tube type has a predetermined current rating, comprising:

means adapted to receive the vacuum tube to be tested, such means including a pair of terminal means for making electrical contact with the terminals of the heater element of the vacuum tube to be tested;

a source of alternating current;

a plurality of fixed capacitors of different capacitive values, each having a single fixed value of capacitance and having a reactance value at the frequency of the alternating current which is 'much greater than the rated operating resistance of the heater elements of any types of vacuum tubes intended to be tested; and

electrical switch means for respectively connecting the source of alternating current and one of the capacitors in series between the pair of terminal means References Cited by the Examiner UNITED STATES PATENTS 11/1923 Beetem 315107 8/1927 McCullough 315105 Voorhoeve 315227 Fisher 315-227 Bird 313--235 Bird 315227 Klutke M 315105 X Sulzer 315105 X DAVID J. GALVIN, Primary Examiner. RALPH G. NILSEN, Examiner. 

1. IN TUBE TESTER APPARATUS FOR TESTING VARIOUS TYPES OF VACUUM TUBES HAVING HEATER ELEMENTS OF RESPECTIVE DIFFERENT CURRENT RATINGS, A CIRCUIT FOR SUPPLYING A SUBSTANTIALLY CONSTANT CURRENT TO THE HEATER ELEMENT OF A VACUUM TUBE UNDER TEST HAVING A PREDETERMINED CURRENT RATING COMPRISING: A SOURCE OF ALTERNATING CURRENT; A PLURALITY OF FIXED REACTANCE ELEMENTS OF DIFFERENT VALUES, EACH HAVING A SINGLE FIXED REACTANCE VALUE AT ANY GIVEN OPERATING FREQUENCY; SWITCH MEANS FOR SELECTIVELY CONNECTING THE ALTERNATING CURRENT SOURCE AND ONE OF THE REACTANCE ELEMENTS IN SERIES WITH THE HEATER ELEMENT OF THE VACUUM TUBE BEING TESTED; AND THE SELECTED REACTANCE ELEMENT HAVING A REACTANCE VALUE AT THE FREQUENCY OF THE ALTERNATING CURRENT WHICH IS MUCH GREATER THAN THE RATED OPERATING RESISTANCE OF THE HEATER ELEMENT OF THE VACUUM TUBE UNDER TEST WHEREBY THE ELECTRICAL CURRENT SUPPLIED TO THE HEATER ELEMENT WILL BE OF A DESIRED PREDETERMINED VALUE WHICH IS SUBSTANTIALLY INDEPENDENT OF THE MAGNITUDE OF THE ACTUAL RESISTANCE OF THE HEATER ELEMENT OVER A WIDE RANGE OF RESISTANCE VALUES. 